This invention relates to electric machines in general and more particularly to a synchronous electric machine with salient poles in its rotor.
Synchronous electric machines with salient poles in their rotors which are supported by a ring body built up from segments, called a lamination chain, which in turn is fastened via a disc shaped or star shaped hub member to the rotor shaft, are described, for instance, in U.S. Pat. Nos. 3,128,403 and 3,529,193. The lamination chain absorbs the centrifugal forces occurring at the salient poles and is thereby radially enlarged. This enlargement can reach radial amounts of 10 to 15 mm especially in slow running hydro-electric generators with a rotor diameter of up to 20 m or more.
If such enlargement are to be taken up by the customary shrink connection between the hub member and the lamination chain without lifting of the lamination chain, the very large shrinkage forces occurring when the machine is standing still bring with them the danger of buckling of the hub arms or hub discs, or the design of the hub, if it is laid out in view of these large shrinkage forces, becomes entirely uneconomical. It is therefore customary to choose the shrink fit so that the lamination chain is lifted from the hub only at approximately 110 to 120% of the nominal speed of rotation. Above this speed, the ring body of the lamination chain expands freely and is constrained only by tangential wedges.
Because of the low flexural stiffness of lamination chains with a large diameter, a deviation of the lamination chain from the circular shape can come about due to unavoidable asymmetrical weight distributions at the circumference, due to nonuniform wedging or due the uneven magnetic pull. In an extreme case this can lead to bridging the air gap, which is relatively small in synchronous machines with such a large diameter. It is therefore an object of the invention to make sure that, if such large enlargements occur when the maximum overspeed is reached, the circular shape of the lamination chain as well as its concentric position are preserved.